June 6, 2020
Pollination is the process by which flowering plants reproduce. Pollen, produced by the male flower part, must move to the female flower part to make a seed. Not all flowering plants are alike. Some have complete flowers with both male and female parts. Others are incomplete with either male or female flowers. Whatever the flower structure, the goal is to make seed. Being rooted in place, the majority of plants need pollinators to transfer pollen. And so begins the love affair of plants and pollinators.
Exploring the connections between plants and pollinators reveals close interdependence and long co-evolution supported by fossil evidence. To entice pollinators, plants developed irresistible rewards. Scientists observing this love affair of plant and pollinator noted the interactions resulted in adaptations on the part of both the flowering plant and the foraging pollinator. To attract pollinators, flowering plants developed sets of traits including color, shape, fragrance, nectar and pollen rewards. For bees, the most important flower traits include yellow, blue and violet petals with UV nectar guides. These floral markings reflect light seen by bees, but not humans and guide the bees to nectar and pollen sources. Hummingbirds usually search for scarlet, red, orange or white funnel-shaped flowers with lots of nectar. Bees and birds build nests for their young. Butterflies and moths choose instead to lay their eggs on host plants. When the eggs hatch, caterpillars eat the leaves of the host plant, pupate and a butterfly emerges. Pollinators feed many times a day correlating benefits with specific plants. Economic foraging results in floral constancy further encouraged when like plants are planted in patches.
Not long ago, my husband and I were browsing in the Cambridge University Bookstore; he in economics and I in horticulture. Turning the pages of Darwin’s “The Effect of Cross and Self-Fertilisation in the Vegetable Kingdom”, I realized Darwin was still working on natural selection twenty years after publishing “Origin of the Species”. We left to walk through Cambridge Botanic Garden’s Pollinator Garden, a demonstration garden about plant and pollinator connections. It turns out ninety percent of insects can reproduce and eat only on the plants with which they share an evolutionary history. Species diversity and preservation is important not just because it creates stability but also, as E.O. Wilson reminds us, a world without insects is a world without humans. Here in Marin, visit a native plant pollinator garden at the Bay Model in Sausalito. Planted by the California Native Plant Society (CNPS) in 2019, the garden is a treasure. CNPS restoration projects include protecting a rare buttercup in Lucas Valley threatened by invasive pennyroyal and the rare Jewelflower in Old St. Hilary’s Preserve threatened by inconsistent rain and invasive broom, fennel, star thistle, and rye and oat grasses. To learn more, go to www.cnps.org.
Plant and pollinator connections can be complicated. For example, a California native butterfly, Edith’s Checkerspot, lays its eggs on Dwarf Plantain, a native herb that grows in serpentine soil. When eggs hatch, caterpillars devour the plantain, crawl to a second host plant, purple owl’s clover, and lastly Indian paintbrush. Once the host plants are devoured, the caterpillars crawl under dead leaves and hibernate for seven months until native plants reemerge and caterpillars complete their lifecycle. As habitat declined, a subspecies, Bay Checkerspot, disappeared from Marin. There are some remaining habitat areas around Santa Clara.
Also declining are species of generalist foragers like bumble bees. UC Riverside scientist Hollis Woodard who studies bumble bees as crop pollinators, reminds us if we want tomatoes, we need to bring back bumble bees. Scientists hope to restore bumble bee populations by seeding grasslands with nectar plants like clover and wild buckwheat. Evidence of the love affair of plants and pollinators surrounds us. Knowledge of specialization can help us rebuild declining insect populations. Gardening ecologically with natives blooming throughout the seasons, we foster the plant and pollinator love affair.